Fibroblast growth factor 21 (FGF21) is an emerging therapeutic target, with several major pharmaceutical companies developing FGF21-based compounds for the treatment of obesity, dyslipidemia, and diabetes. Nevertheless, the physiological role of this hormone in the context of normal metabolic homeostasis remains unclear, increasing the risk of unintended or off-target clinical outcomes. Moreover, to our surprise, almost nothing is known about FGF21 physiology or pharmacology in females. FGF21 was originally described as a hepatokine that is secreted into circulation in response to a prolonged fast, and it is thought to contribute to aspects of the adaptive starvation response. However, a growing body of literature now indicates a more nuanced role. Simply decreasing the abundance of dietary protein, relative to carbohydrate in particular, increases plasma FGF21 in rodents and in humans? despite normal or even increased caloric intake. Thus, circulating FGF21 signals a relative deficit of dietary protein, rather than calories per se, in both mice and in humans. These new findings represent a shift in our understanding of FGF21 physiology, and suggest that in addition to its well-studied effects on carbohydrate and lipid metabolism, FGF21 may also regulate protein and amino acid homeostasis. Our preliminary data are consistent with this possibility, and moreover suggest the metabolic effects of FGF21 are sex-dependent. Our overall objective, therefore, is to determine the influence of FGF21 on whole-body protein and amino acid metabolism in both male and female mice. Specifically, we hypothesize that FGF21 acts via distinct behavioral, neuroendocrine, and biochemical mechanisms to conserve systemic amino acids. We will test this overall hypothesis in three Specific Aims. Aim 1 uses acute and chronic models of dietary macronutrient preference, to determine the role of FGF21 for control of dietary protein intake by males and females. Aim 2 uses isotopic tracer-assisted studies of skeletal muscle turnover, to determine the effect of FGF21 on protein degradation and synthesis in males and females. Aim 3 uses comprehensive energetic, transcriptional, and metabolomics profiles, to determine the role of FGF21 to regulate hepatic substrate utilization in males and females. Upon successful completion of this project, we expect to identify a novel, sex-dependent, role for FGF21 in the control of whole-body protein and amino acid homeostasis. We expect this work to be of broad interest, not only for the study of obesity, dyslipidemia, and diabetes, but also for related fields like aging, cancer, development, musculoskeletal disorders, and nutrition. Importantly, these studies will be among the first to establish sex-dependent outcomes in FGF21 physiology, providing key information needed for targeted delivery of FGF21-based therapy.